In a semiconductor device with a case housing filled with gel, the case housing and a heat radiation plate are adhesively bonded to each other with adhesive agent. This makes it possible to prevent moisture intrusion from a joint portion or leakage of the gel to the outside. However, in order to secure a certain volume of the adhesive agent coated on the heat radiation plate, the case housing encloses the heat radiation plate therein. Accordingly, the size of the heat radiation plate must be made smaller than that of the case housing. Furthermore, when there is no step portion both on a case housing side and on a heat radiation plate side (see Patent Literature 1, for example), there is a concern that a sealing function of the adhesive agent would be impaired because the adhesive agent is crushed and the volume of the adhesive agent itself is lost.